A circulator is a fundamental component of RF and microwave equipment such as, for example, transmitter multi-couplers that allow radio transmission sites to operate reasonably free of interference. Circulators such as ferrite circulators typically include at least three terminals or ports at which an external waveguide or transmission line connects to the circulator. A signal entering in any port can be transmitted only to the next port in rotation. Moreover, a circulator is a three (or four) terminal, non-reciprocal device that permits RF or microwave energy to flow between two adjacent ports in only one direction.
When only two terminals of the circulator are used, an isolator may be formed. Thus, for example, if one port of a three-port circulator is terminated in a matched load, the circulator may operate as an isolator, allowing signals to travel only in one direction between the two remaining ports.
Impedance matching is an important consideration when connecting a circulator/isolator to external equipment. In the past, circulators/isolators have generally been provided with a junction impedance of 50 ohms. The provision of a stable and precise 50 ohm junction impedance for each port of the circulator/isolator, has been adopted as an industry standard so that each port can have a predictable impedance. However, a byproduct of this standard has been that in many cases, loads that may need to be served could have impedances of something other than 50 ohms.
Given that the natural impedance of a circulator/isolator is much less than 50 ohms, most circulators/isolators employ some form of impedance transformation circuitry to generate a final impedance of 50 ohms at the connector port. Accordingly, when an external device having an impedance of less than 50 ohms is to be coupled to the circulator/isolator, further impedance transformation must be employed to transform the impedance back down to the impedance of the external device. The result may be a complex series of cascading impedance transformers that may add to the cost and complexity of the devices.